The invention relates to a piercing mandrel for piercing heated round blocks of metal for the production of seamless pipes, having a piercing mandrel nose and a piercing mandrel main body which can be connected to a mandrel bar.
It is generally known that seamless pipes are produced by means of a method, in which starting material which is cylindrically formed and heated in a rotary hearth furnace, so-called round blocks, is formed in a piercing mill using an axially fixed inner tool consisting of a piercing mandrel and a mandrel bar to produce a pipe-shaped hollow block.
In order to form a seamless hollow block pipe, after being grasped by the piercing mill the block is transported in helical line fashion through the piercing mill and thus over the axially fixed inner tool and in the subsequent course of the method is wall-ironed to form a seamless pipe. The hole is produced by virtue of the fact that the round block is advanced by the skewed rollers and is urged over a piercing mandrel. The piercing mandrel itself typically has a diameter which increases continuously starting from the tip.
The piercing mandrel which typically consists of a highly heat-resistant tool steel not only has the task of actual piercing but also the task of welding any material fractures, smoothing the inner surface of the resulting hollow block and bringing the wall thickness thereof to the desired dimension in the most uniform manner possible.
Since the piercing mandrel has to perform its work under the influence of rolling heat, it is subject to extreme loadings and only has a limited service life. Since the invention of the skew rolling method, efforts have been made to extend the service life of the rolling mandrels, to save costs and to improve the quality of the rolled pipes. If the piercing mandrel, which is also referred to as a rolling mandrel, is coming to the end of its tool life, the piercing mandrel can also lose its shape and its surface can become damaged or the material thereof can become fractured. At the same time, this also signifies a deterioration in pipe quality, in particular by reason of defects on the inner surface of the hollow block and a non-uniform wall thickness.
In this case, particular problems are posed by pipe materials which are difficult to work, such as chromium-containing materials, having more than 5 wt. % chromium, which place particular thermal and mechanical loadings upon the piercing mandrel and in particular the mandrel tip, so that even after a small number of piercing procedures the mandrel tip has traces of wear which quickly lead to detrimental changes in the geometry of the mandrel and thus to internal defects and geometry deviations in the hollow block.
Over the course of time, many measures have been proposed which were intended to reduce heat introduction into the mandrel and increase the durability. Examples of this include the production of piercing mandrel tips from particularly heat-resistant materials, such as e.g. engineering ceramics, the coating of the piercing mandrel surface with additional materials, controlled oxidation of the surface, frequent replacement of the mandrels in conjunction with water-spray and immersion cooling, and internal cooling of the piercing mandrels with water through the mandrel bar. An overview of the prior art is provided in Japanese laid-open document JP 03204106 A, German patent specification DE 196 36 321 C1 and European laid-open document EP 2 404 680 A1.
In order to improve the durability of the piercing mandrel and to avoid internal defects in the hollow block, it was attempted in accordance with European laid-open document EP 2 404 680 A1 to achieve an improved durability of the piercing mandrel by means of an optimised geometry of the piercing mandrel tip in conjunction with an iron oxide coating. At the same time, it is proposed to avoid friction-induced surface defects on the piercing mandrel by spraying a cooling lubricant through the piercing mandrel tip into the resulting hollow block. For this purpose, the cooling lubricant is directed through the main body of the piercing mandrel via outlet openings, which extend obliquely into the piercing mandrel tip, into the hollow block. To this end, the piercing mandrel tip consists of a cylindrical portion and a hemispherical tip, wherein the outlet openings are disposed in the transition region between the cylindrical portion and the piercing mandrel main body. Disadvantages in this case include the still inadequate cooling of the piercing mandrel tip itself and the inclined position of the outlet openings for the coolant at the piercing mandrel tip which on the one hand is complex to produce and on the other hand no longer reliably prevents closure during the piercing procedure through contact with the material of the round block. Another disadvantage is the one-piece design of the piercing mandrel which in the event of wear requires the entire piercing mandrel to be replaced.
In order to lower the cost of replacing worn piercing mandrels, it has also been proposed in German laid-open document DE 100 24 246 A1 to form the piercing mandrel in two parts with a mandrel nose and a piercing mandrel main body, which can be connected thereto, and for the purpose of increasing durability, to produce the mandrel nose and the main body from different materials corresponding to the thermal and mechanical loadings, and where required to replace only the piercing mandrel tip. A disadvantage in this case is that the mandrel tip is not cooled and the durability is still inadequate specifically in the case of materials which are difficult to work.
Furthermore from the European patent document EP 1 961 497 B1 a further piercing mandrel of a piercing mill is known. This piercing mandrel shows a piercing mandrel nose at a piercing mandrel main body. The piercing mandrel nose is conically broadened seen in the direction of the piercing mandrel main body.
All of these measures have hitherto been unsuitable to effectively reduce the thermal and mechanical loading upon the piercing mandrels during skew rolling such that from an economical and qualitative aspect, sufficiently high durability could be achieved in particular in the case of materials which are difficult to work.